Organic EL panel

ABSTRACT

Pixels for three colors, namely, R, G, and B, are arranged in a stripe manner. Two data lines DL are placed in a space between pixel columns for every other pixel column, and a power supply line is placed in a space between the pixel columns, where a data line is not placed. Pixel columns for colors with the best current efficiency and with the least current efficiency share a single power supply line, while pixel columns for colors with intermediate current efficiency share another power supply line.

CROSS-REFERENCE TO RELATED APPLICATIONS

The entire disclosure of Japanese Patent Application No. 2003-337924including specification, claims, drawings and abstract is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a stripe-type organicelectroluminescence (EL) panel which comprises at least three kinds ofpixels, in which the pixels, each including an organic EL element foremitting light of a particular color, are arranged in a matrix such thatpixels for emitting the same color light are arranged in a column.

2. Description of the Related Art

Organic electroluminescence (hereinafter referred to as EL) panels havebeen receiving attention as one of the next generation flat displays totake place the liquid crystal displays. In the display panels(hereinafter referred to as an organic EL panel), the color of lightemitted by each pixel can be determined depending on the kind of lightemitting material used to form an organic light emitting layer for thepixel. Thus, pixels for different color light are formed so that RGBdisplay can be realized.

In this organic EL panel, bright display is achieved by increasing theamount of current supplied to each organic EL element. However, increaseof a current amount results in reduction of the service life of anorganic EL element. Therefore, instead of increasing a current amount,it is desired to ensure the largest possible area for a light emittingregion, or an aperture ratio, in each pixel.

A larger aperture ratio enables brighter display while suppressingcurrent supplied to the organic EL element to a relatively small amount(See Japanese Patent Laid-open Publication No. 20001-290441). Increaseof an aperture ratio means increase of the proportion of the lightemitting region of an organic EL element relative to the entire area ofeach pixel.

Here, in an organic EL panel of an active type, at least two TFTs areprovided in each pixel to drive the concerned organic EL element.Moreover, a data line for supplying luminance data to each pixel and apower supply line for supplying driving current to the organic ELelement of each pixel are provided in a column direction, while aselection line (gate line) for selecting a pixel is provided in the rowdirection. Therefore, the areas where these wirings are placed are notavailable for use as a light emitting region.

Further, each pixel is provided with a selection TFT, to be turned on oroff by the selection line, for supplying a voltage at the data line to aholding capacitor, and a driving TFT for supplying a driving currentaccording to the voltage held at the holding capacitor, from the powersupply line to the organic EL element. Therefore, the areas where theTFTs are placed are also not available for use as a light emittingregion.

This limitation of an area available for a light emitting region leadsto a problem of a reduced aperture ratio.

SUMMARY OF THE INVENTION

As described above, according to the present invention, one power supplyline is shared by two pixel columns. When two power supply lines arecombined into one line, basically, a line width which is twice the linewidth of a single line is necessary. However, as the line width of asingle line includes an indispensable margin for each line, when twolines are combined into one line, the margin for one line becomesredundant, in other words, omissible. Therefore, the line width of oneline resulting from two lines combined becomes smaller than that whichresults from simple addition of two lines.

Moreover, two lines of power supply and data lines are generally laidbetween pixel columns, and, in such a case, according to a design rule,a predetermined space must be ensured between the laid lines. However,by sharing a single power supply line for two pixel columns, the spacewhich would otherwise be required between two wirings can be eliminated.This also contributes to reduction of the distance between pixelcolumns, so that the aperture ratio can be improved.

Further, the light color of a pixel column to be caused to share asingle power supply line is appropriately selected such that the maximumcurrent amounts for the respective power supply lines become close toone another. This can prevent use of a very wide line, compared to otherlines, for a power supply line to be shared, and efficient currentsupply can thus be maintained.

When four kinds of pixels for R, G, B, and W colors are used, pixelcolumns for two colors may share one power supply line so that powersupply lines can be arranged in a balanced manner. Specifically, when apixel column for light color with the maximum current efficiency andthat with the minimum current efficiency share a single power supplyline, while pixel columns for other light colors share another powersupply line, lines having a substantially constant width can be used forthe respective power supply lines.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a diagram schematically showing a plane structure of anembodiment of the present invention;

FIG. 2 is a diagram showing a plane structure of the embodiment; and

FIG. 3 is a cross sectional diagram showing essential portions in theembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, an embodiment of the present invention will bedescribed with reference to the accompanied drawings.

FIG. 1 schematically shows a structure of an embodiment of the presentinvention. A single data line DL is provided for each pixel column andmounted such that two data lines DL are placed between pixel columns forevery other pixel column. A power supply line PL is placed in a spacebetween pixel columns, where a data line DL is not mounted. A gate lineGL is mounted between pixel rows.

In each pixel, a selection TFT 1, a driver TFT 2, a holding capacitor 3,and an organic EL element 4 are provided. In this example, the selectionTFT 1 is a p-channel TFT, and connected, via its source, to a data lineDL, via its drain, to the gate of the driver TFF 2, and via its gate, toa gate line GL.

The driver TFT 2 is also a p-channel TFT, and connected, via its source,to a power supply line PL, and via its drain, to the node of the organicEL element 4. The cathode of the organic EL element 4 is grounded. Thegate of the driver TFT 2 is connected to one end of the holdingcapacitor 3, the other end of which is connected to the holdingcapacitor line SL.

With this structure, by setting the gate line GL at a low level (L), theselection TFTs 1 in the relevant pixel row are turned on. Then, whenpixel data for the respective pixel columns are supplied to the datalines DL for the respective pixel columns while the selection TFTs arein an ON state as described above, the gates of the respective driverTFTs 2 of the respective pixels are set at the voltage of the pixeldata, and the voltage is held in the respective holding capacitors 3.

Thereafter, appropriate current according to the supplied pixel data issupplied from the power supply lines PL via the driving TFTs 2 to therespective organic EL elements 4 of the respective pixels, so that lightemission according to the pixel data is achieved.

In this embodiment, each pixel emits light of either red (R), green (G),blue (B), or white (W), and the pixels for the same color light arearranged in the same column, that is, a stripe arrangement. As a result,pixels for R, B, G, and W light are arranged in the row direction. Inthis example, pixels for R and B light have a substantially identicalwidth, while pixels for G light have the largest width and pixels for Wlight have the smallest width.

A power supply line PL is provided running either between R and B pixelcolumns or between G and W pixel columns. That is, a single power supplyline PL is shared by either R and B pixel columns or by G and W pixelcolumns.

The size of each pixel is determined based on the current efficiency ofthe relevant organic EL element 4. “Current efficiency”, rephrased as anexternal quantum efficiency, refers to the amount of light emitted for aunit current, and, in this embodiment, corresponds to the maximumcurrent amount used for display. Larger necessary maximum current meanspoorer current efficiency. The current efficiency depends on the kind oforganic light emitting material, and so forth, used to form an organicEL element 4.

In this example, an organic EL element 4 for W light has the highestcurrent efficiency, while an organic EL element 4 for G light has thelowest current efficiency. For color balancing in full-color display, anorganic EL element 4 for a color with low current efficiency requires acurrent of a larger amount.

Here, as the service life of an organic EL element 4 is governed by therelevant current density, a constant current density is desired to bemaintained with the respective organic EL elements 4. For this reason,an organic EL element 4 for a color with poor current efficiency isdesigned to have a larger area so that a constant current density can bemaintained with the respective organic EL elements 4.

A pixel column for W light with the best current efficiency and a pixelcolumn for G light with the poorest current efficiency share a singlepower supply line PL, while pixel columns for R and B light withintermediate current efficiency share another power supply line PL. Withthis arrangement, substantially close amounts of current flow in therespective power supply lines PL.

Here, the line width is determined based on the maximum current amountand a very wide line cannot be employed due to the upper limit of theline width. By balancing the current amounts as in this embodiment, itis possible to cause close amounts of current to flow in the powersupply lines PL running between the pixel columns for R and B light andbetween pixel columns for G and W light, respectively, so that effectivecurrent supply can be achieved.

Here, a conventional structure requires one power supply line PL and onedata line DL for each pixel column, and these two lines are arranged ina space between pixel columns. In general, according to a design rule, aspace of about 4 μm must be ensured between two parallel runningwirings.

In the case where two power supply lines PL, each having the width ofabout 10 μm, for example, are arranged between pixel columns, the totalline width of these two lines is generally 24 μm. In this embodiment,however, where one, instead of two, power supply lines PL are used, itis known that the entire line width is as small as about 15 μm.

This is because, as the 10 μm width of each line includes anindispensable margin for one line, when two lines are combined into oneline, the margin for one line becomes redundant, that is, omissible. Inaddition, as the space, which is 4 μm here, between two lines is alsoomissible, the total width of 9 μm can be eliminated, as compared to acase where two separate lines are used. Consequently, only a smallerarea is necessary for installation of the wiring, and accordingly alarger aperture ratio can be ensured.

It should be noted that the superiority order of current efficiencyamong colors W, R, B, and G is not limited to the one described in thisembodiment, that is, W, R, B, and G, but may vary depending on the lightemitting material used for the respective colors.

For three color light emission for R, G, and B, pixels for R light withthe best light emitting efficiency may use a dedicated power supplyline, while pixels for G and B light with lower efficient light emittingefficiency may share another power supply line.

FIG. 2 shows a specific arrangement of pixels. A selection TFT 1includes a semiconductor layer. A part of a gate line GL project over achannel region 1 c of the selection TFT 1 to constitute a gate electrode1 g. A source 1 s of the selection TFT 1 is connected to a data line DLwhich is located above the selection TFT 1, via a contact.

The semiconductor layers for the source 1 s extend so as to constitute acapacitor electrode 3 a, which, together with a holding capacitor line(not shown) arranged opposed to the capacitor electrode 3 a, constitutea holding capacitor 3.

The capacitor electrode 3 a is connected, via a contact, to a gateelectrode 2 g of a driver TFT 2. The gate electrode 2 g runs straight inparallel to (and along with) the power supply line PL. It should benoted that the gate electrode 2 g is located partly below the powersupply line PL.

The semiconductor layer for the driving TFT 2 (portions 2 s, 2 c, 2 d,all together referred to by reference numeral 2 p, described later)extends upward (in the drawing) from a contact formed on a portion ofthe power supply line PL, which projects into the pixel region, until itbends at a right angle such that the driving TFT 2 is formed in an L orinvert-L shape. The other end of the driver TFT 2 is connected via acontact to the anode of the organic EL element 4, which is located abovethe driver TFT 2.

In this example, the driver TFT 2 is a p-channel TFT. The portion of thedriver TFT 2, which is connected to a power supply line PL, is a source,while the portion thereof which is connected to the anode of the organicEL element 4 is a drain. The gate electrode of the driver TFT 2 isformed so as to cover the portion of the semiconductor layer, locatedbetween the source and drain, where no impurities are doped.

As described above, when the driver TFT 2 is shaped like an L orinverted L, at least a portion of the gate electrode 2 g can be placedbelow the power supply line PL. That is, through utilization of thespace below the power supply line PL, the aperture ratio can beincreased.

Further, as the contact with the organic EL element 4 (an area in thevicinity of the area with reference numeral 2 s in the drawing) islocated in the center portion of each pixel region, the gate electrode 2g can be formed straight. This can prevent reduction of the apertureratio due to a gate electrode 2 g detouring around the contact.

Still further, as the respective pixel regions have a constant height inthis embodiment, the gate line GL can be laid straight. Yet further, thepower supply line PL and the data line DL also can be laid straight,even though the width of pixel regions varies, as the pixels arearranged in stripe.

Yet further, the shape of a light emitting region inside each pixelregion is modified for efficient arrangement of itself. For example, ina G pixel with a wider pixel region, a holding capacitor 3 is arrangedbeside the selection TFT 1, leaving sufficient space for the lightemitting region to extend upward (in the drawing) within the pixelregion. In this manner, the pixel region is efficiently utilized.

All driver TFTs have the same size, and a structural relationshipbetween a driver TFT and a concerned power supply line PL is identicalfor all pixels. That is, in all pixels, a contact between a driver TFT 2and a power supply line PL and a contact between the driver TFT 2 and anorganic EL element are identically positioned in the horizontaldirection (in the drawing), as well as in view of the concerned powersupply line PL. This arrangement can readily achieve constant currentsupply capacities of the respective driving TFTs 2.

It should be noted that, in FIG. 2, the transparent electrode part ofthe organic EL element 4 is indicated by a two-dot line and, simplifyinterpretation of the drawing, is shown in a smaller size.

FIG. 3 is a cross sectional view showing a structure of a light emittingregion and a driver TFT of a single pixel along line X-X in FIG. 2, thatis, along a line bending at a right angle along the L-shaped driver TFT2.

As shown, a buffer layer 11 comprising SiN and SiO₂ laminated layers isformed over the entire surface of a glass substrate 30, and apoly-silicon semiconductor layer (an active layer) 2 p is formed thereonin a predetermined area (an area for a TFT).

Further, covering the entire surface of the active layer 2 p and thebuffer layer 11, a gate insulating film 13 is formed. The gateinsulating film 13 comprises, for example, SiO₂ and SiN laminatedlayers. On the gate insulating film 13 and in an area above the channelregion 2 c, a gate electrode 2 g is formed using Cr, for example.

Then, using the gate electrode 2 g as a mask, impurities are doped intothe active layer 2 p, whereby a channel region, where no impurities aredoped, is formed in the middle of the active layer 20 below the gateelectrode 2 g, and source and drain regions 2 s and 2 d, whereimpurities are doped, are formed on both sides of the channel region 2 cin the active layer 2 p.

Further, covering the entire surface of the gate insulating film 13 andthe gate electrode 2 g, an interlayer insulating film 15 is formed.Then, contact holes are formed, piercing through the interlayerinsulating film 15, above the source region 2 s and the drain region 2d, respectively, and a source electrode 53 and a drain electrode 26 areformed on the surface of the interlayer insulating film 15 through therespective contact holes. The source electrode 53 is connected to apower supply line (not shown). The thus formed driving TFT may be eithera p-channel TFT, as is in the above example, or an n-channel TFT.

Still further, a planarization film 17 is formed covering the entiresurface of the interlayer insulating film 15. Then, a transparentelectrode 61 is formed on the planarization film 17, for serving as ananode of the organic EL element 4. Another contact hole is formed abovethe drain electrode 26, piercing through the planarization film 17, andthe drain electrode 26 is connected to the transparent electrode 61through the contact hole.

It should be noted that the interlayer insulating film 15 and theplanarization film 17 are generally made using an organic film such asacrylic resin, and may alternatively be made using an inorganic film,such as TEOS. The source electrode 53 and the drain electrode 26 areformed using metal such as aluminum, while the transparent electrode 61is generally formed using an ITO.

Further, an organic layer 65, which comprises a hole transport layer 62,an organic light emitting layer 63, and an electron transport layer 64,is laid on the transparent electrode 61. The hole transport layer 62 andthe electron transport layer 64 cover the entire surface of the pixels.The organic light emitting layer 63 is slightly larger than the lightemitting region. Further, an opposing electrode 66, comprising metal,such as aluminum (AL), is formed over the entire surface of the pixels,to serve as a cathode.

In this structure, a planarization film 67 is formed surrounding thetransparent electrode 61 and below the hole transport layer 62. Theplanarization film 67 defines a part of the hole transport layer 62,which is in direct contact with the transparent electrode 61, and thepart constitutes a light emitting region for a concerned pixel.

It should be noted that the planarization film 67 is generally madeusing an organic film made of acrylic resin or the like, and mayalternatively be made using an inorganic film, such as TEOS.

The hole transport layer 62, the organic light emitting layer 63, andthe electron transport layer 64 are made using material which isgenerally used for an organic EL element. The color of emission lightdepends on the material used to form the organic light emitting layer63, which is generally dopant. For example, NPB is used for the holetransport layer 62; TBADN+DCJTB is used for an organic light emittinglayer 63 for red light; Alq3+CFDMQA is used for an organic lightemitting layer 63 for green light; TBADN+NPB is used for an organiclight emitting layer 63 for blue light; and Alq3 is used for theelectron transport layer 64.

With the above-described structure, when the driver TFT 2 is turned onaccording to the voltage set in the gate electrode 2 g, current issupplied from the power supply line via the transparent electrode 61 tothe opposing electrode 66, and light emission according to the currentis caused in the organic light emitting layer 63. The emitted lightpasses through the transparent electrode 61, the planarization film 17,the interlayer insulating film 15, the gate insulating film 13, and theglass substrate 30, to be emitted downward (in the drawing).

1. An organic EL panel of a stripe type, in which at least three kindsof pixels, each including an organic EL element for emitting light of aparticular color, are arranged in a matrix such that pixels for the samecolor are arranged in a column, wherein a single power supply line isplaced between columns of pixels for two particular colors, forsupplying power to the organic EL elements of the pixels in two columns,and the two particular colors are selected based on current efficiencyin light emission by the organic EL elements for respective lightcolors, such that a maximum current amount of a corresponding powersupply line is set close to that of other power supply lines.
 2. Theorganic EL panel according to claim 1, wherein light emitting colors ofthe pixels include R (red), G (green), B (blue), and W (white), one ofthe power supply lines is shared by pixels in a column for a light colorwith maximum current efficiency in light emission by the organic ELelement and pixels in a column for a light color with minimum currentefficiency, and another power supply line is shared by pixels in columnsfor other light colors.
 3. The organic EL panel according to claim 2,wherein the one of the power supply lines is shared by pixels for W andG light, and the other power supply line is shared by pixels for W and Glight.
 4. The organic EL panel according to claim 1, wherein each pixelhas a holding capacitor for holding a gate voltage of the drivetransistor, the holding capacitor is placed on a side of one end of eachpixel in the column direction, the organic EL element is placed on aside of another end of the pixel in the column direction, and the drivetransistor is placed between the organic EL element and the power supplyline.
 5. The organic EL panel according to claim 4, wherein the powersupply line has a projected portion projecting toward a side of thepixel, and the projected portion is connected to the drive transistor.6. The organic EL panel according to claim 4, wherein a gate electrodeof the driving transistor is provided running along the power supplyline, and one end of the gate electrode is connected to one electrode ofan auxiliary capacitor.